The Resilient Packet Ring (RPR) technology is a packet transmission technology, which combines the advantages of the Ethernet, ATM, and Synchronous Digital Hierarchy. The RPR makes use of the QoS feature of the ATM, the statistical multiplexing and simplicity of the Ethernet, and the protection feature of the SDH to transmit data traffic efficiently and safely.
The RPR is a duplex-fiber ring structure, as shown in FIG. 1, and each fiber can transmit data packets. For distinction, one ring is called “inner ring” and another is “outer ring.” During the RPR operation, the data packet is transmitted in one direction and at the same time the control packet is transmitted in the opposite direction. In this way, the two fibers are used simultaneously, and this will speed up the control signal transmission.
The characteristics of the RPR technology can be summarized as follows:
1. Topology discovery and protection. Based on the network topology structure, the RPR selects a shortest path for data transmission. The inner ring and outer ring can transmit data frame simultaneously, so efficiency is doubled compared to the SDH network. The RPR has the ability to protect a single span (node or filter) failure within 50 ms.
2. Spatial reuse. RPR deploys the Destination-Stripping protocol; its unicast packets are stripped at their destination, unlike the FDDI, which deploys Source Stripping protocol, where circuits consume bandwidth around the whole ring, and RPR allows bandwidth to be used on idle spans.
3. Fairness algorithm. RPR supports fairness allocation of bandwidth with a fairness algorithm, which provides total bandwidth fairness allocation and part bandwidth fairness allocation. With this fairness algorithm, the QoS can be guaranteed.
At present, two schemes as shown in FIG. 2 are applied to transmit data traffic in synchronous digital network equipment. The first scheme is the EOS. The data frame from the Ethernet interface is encapsulated based on protocol and made rate adoption directly, then is mapped to the Virtual Container of synchronous digital network and is transmitted in a point-to-point manner though nodes of the SDH. The second scheme includes the step of inserting a Layer 2 Switch module in a Ether Ring after the Ethernet interface, and transmitting the data that has been processed by the Layer 2 Switch module on the synchronous digital network. Since the multi-service transport platform is based on the SDH/SONET and supports the Layer 2 Switch module, the data frame exchange between Ethernet interface on the user side and the VC channel on the network side can be performed based on Ethernet linkage layer.
The first scheme has the following disadvantages.
(1) It does not support bandwidth statistical multiplexing. The bandwidth of each node in the SDH is fixed and can only be used by its own node, so if its own node is idle, other nodes cannot use the idle bandwidth.
(2) Only point-to-point connection is implemented so that traffic convergence of point to multi-point cannot be implemented. Since the SDH/SONET implements only point-to-point transmission, the EOS scheme can only implement point-to-point transmission. Therefore, traffic convergence cannot be implemented at a convergence node.
(3) Implementation is complex. Due to the connection-orientation characteristic of the SDH/SONET and uncertain bandwidth of EOS, SDH/SONNET has several selectable virtual concatenations, such as VC12, VC3, or VC4, and implementation of them is complex when there are many nodes and connection-oriented characteristic exists, N×N connection is applied.
The second scheme overcomes disadvantages of the first scheme, but it has its own disadvantages, too.
(1) Bandwidth is unfairly distributed. In the second scheme, there is no access bandwidth control mechanism at each node of the Ether Ring, so each node may compete for the ring bandwidth, and the source node that is nearer the destination node takes more bandwidth. This is unfair for other nodes in the Ether Ring.
(2) QoS is worse. Every node takes store-and-forward mode for traffic transmission, so jitter and latency-sensitive service, such as VOIP service, cannot have high priority for transmission.
(3) Protection switching duration is long. Once a span fails, only the spanning tree protocol can be used for protection; its protection switching duration is usually several minutes so that protection switching duration within 50 ms cannot be met.